This invention relates generally to road construction apparatus and more particularly to a field configurable device for controlling the position of a road leveling instrument.
As is known in the art, road paving apparatus generally includes a leveling, or grading instrument to provide a smooth surface of asphalt over the road bed. The leveling instrument, often referred to as a screed, is a free floating instrument which is towed behind the paver. More particularly, each of a pair of screed positioning members has a first end coupled the paver and a second end attached to the leveling instrument. As the paver moves, the leveling instrument moves over asphalt deposited by the paver onto the road bed in order to provide a smooth road surface.
As is also known in the art, the thickness of asphalt material on the road bed is increased or decreased by raising or lowering the position of the leveling instrument, respectively. Generally, the first end of each of the pair of screed positioning members is coupled to the paver by hydraulic cylinders. The hydraulic cylinders allow the position, or height of the leveling instrument to be adjusted relative to the road bed. Thus, the movement of the hydraulic cylinders provides corresponding movement in the screed positioning members and consequently, in the position of the leveling instrument. Moreover, the hydraulic cylinders raise or lower the first end of the screed positioning members individually or in concert. For example, if the paved surface on the left side of the paver has a desired thickness but such surface on the right side of the paver is undesirably thin, the hydraulic cylinder disposed on the right side of the paver raises the corresponding screed positioning member. In this way, the position of the right side of the leveling instrument is raised and that of the left side is kept constant in order to achieve the desired asphalt thickness.
As is also known in the art, road paving apparatus often includes a grade control device which controls the hydraulic cylinders and, thus, the position of the leveling instrument above the road surface. The grade control device compares a reference distance (i.e. a preset distance corresponding to the distance between the control device and a reference surface, or datum) to a distance measured between the control device and the datum. In response to the difference between such distances, electrical control signals are generated which control solenoids in the hydraulic cylinders to move the leveling instrument upward or downward accordingly.
Typically, the datum is either a wire disposed at a predetermined distance above the road surface, a ski-like structure which drags along the road surface, or the road bed itself. The reference distance is preset by the operator of the paving equipment when the paver is set up, or initialized, prior to a paving operation. More particularly, the leveling instrument is, initially, manually adjusted to be disposed at a desired distance above the road bed (i.e. corresponding to a desired asphalt thickness). The reference distance is then set, for example, by activating a switch which causes the distance between the control device and the datum to be measured and stored in a memory device. In operation, the grade control device adjusts the position of the leveling instrument to maintain the set reference distance between the control device and the datum. In this way, the level of the grading instrument is maintained at a constant distance from the datum.
As is also known in the art, the distance between the control device and the datum may be measured in various manners. One way of making such a distance measurement is by using a contacting wand. With this arrangement, the wand drags along the datum and is displaced upward or downward depending on the level of such datum. However, the mechanical wand may become caught or jammed on obstacles on the road surface, for example, and thus, may not provide a reliable distance measurement.
An alternate way of measuring the distance between the control device and the datum is to use a non-contacting sonic device. Non-contacting measuring devices are desirable since they have fewer moveable parts and, thus, tend to be more reliable. An example of a grade control device employing sonic sensing is found in U.S. Pat. No. 4,933,853 entitled "Ultrasonic Grade and Auger Control". In the apparatus described therein, a sonic signal is transmitted from the control device downward to the datum and is received by such device after reflecting off the datum. The time between the transmission and receipt of the signal is measured and used to determine the distance between the control device and the datum. More particularly, a first time period is determined by measuring the time between transmission and receipt of a sonic signal directed toward the datum. A second time period corresponds to the time between transmission and receipt of a sonic signal directed toward a target disposed at a predetermined target distance from the control device. The distance between the control device and the datum is then determined by multiplying the ratio of the predetermined target distance (i.e. the distance between the control device and the target) to the second time period by the first time period. The above-described method of measuring the distance between the control device and the datum (i.e. in particular, the use of the target) compensates for changes in air temperature, such changes otherwise causing undesirable variations in the measured distance. The measured distance between the control device and the datum is then compared to the preset reference distance, as mentioned above, to provide an error signal corresponding to the difference between such measured and reference distances. In response to the error signal, electronic signals ar generated to control the hydraulic cylinders which, in turn, control the position or height of the leveling instrument as described above.
The sonic control device is mounted on the screed positioning members (or alternatively two sonic control devices may be used, one mounted on each of the pair of screed positioning members) and may be moved along such member between the screed and the hydraulic cylinder in order to adjust the sensitivity of the device. For example, consider the case where the pivot point of the positioning member is disposed relatively close to the leveling instrument (i.e. relatively far from the hydraulic cylinder). In this case, a relatively large displacement by the hydraulic cylinder will provide a relatively small corresponding displacement of the leveling instrument. Thus, the sensitivity of the grade control device to movement of the hydraulic cylinder is greater when such device is disposed adjacent to the cylinder and this increased sensitivity yields faster response times and reduces overshoot. In this way, the quality (i.e. smoothness) of the road can be adjusted by moving the grade control device along the positioning member.
The sonic control device includes a microprocessor and provides a feature whereby the reference distance, once set, may be adjusted without having to re-initialize the grade control device. Such adjustments may be desirable in situations where special requirements of an area of paving necessitate a slightly thinner or thicker road surface, for example. Furthermore, it may be desirable to adjust the reference distance in situations where the level of the road surface varies significantly between the location of the control device and the location of the leveling instrument. The reference distance may be adjusted, for example, by using a switch in which the increment of adjustment is fixed, or alternatively, by using a rotary knob. The drawback of using a switch is that the fixed increment should be small enough to allow fine adjustments to be made; however, such a small increment may be inconvenient in applications where relatively large adjustments are desired quickly. Furthermore, the rotary knob may be undesirable since the operator generally has no means of knowing the increment of adjustment corresponding to a given angular displacement of the knob until the leveling instrument moves. Such trial and error adjustment may result in overshoot of the leveling instrument with respect to the desired distance to the datum or in excessively slow adjustment due to the operator's caution in turning the knob slowly to avoid overshoot.
Another aspect of the sonic control device is a deadband feature which permits a predetermined range of error signal values to be overlooked by the control device. Stated differently, the grade control device will not adjust the position of the leveling instrument unless the error signal is greater than a predetermined threshold. This feature is particularly desirable in applications where the datum has small ripples in its surface, for example, in the case of a road bed providing such datum. Because the ripples are small, the resulting error signal is small. Thus, by having a deadband feature, such small error signals may be ignored or disregarded. In other words, the hydraulic cylinders, and thus the leveling instrument, will not move or track the small ripples in the road bed datum. As a result, the ripples is the road bed will not cause corresponding ripples in the newly paved surface. However, the predetermined threshold level below which the error signals are ignored is generally pre-programmed into the microprocessor and, thus, cannot be adjusted by the operators of the paving equipment at the work site. Alternatively, such threshold level may be modified by adjusting a potentiometer, for example. However, generally the operator adjusting such potentiometer has no means of determining the actual deadband value (i.e. predetermined threshold level) and, thus, must use a trial and error process to establish the desired deadband.
As is also known in the art, a mechanical support or bracket positions the target at a predetermined, or target distance from the control device. If the target distance were to change prior to the reference distance being set, the preset reference distance would not accurately represent the actual distance between the sonic sensor and the datum. In such circumstances, the control device would still maintain the desired reference distance between the screed and the datum since both the reference distance and the distance measured between the control device and the datum are determined using the target distance. However, the reference distance is generally continuously displayed, for example with an LCD display and thus, the discrepancy between the actual distance between the control device and the datum when the device is initialized and that displayed as the reference distance may cause the operator some concern.